1. Field of the Invention
The present invention relates generally to the treatment of used foundry sands and, more specifically, to a method for removing minute traces of thermally fused foundry bonding clays consisting principally of Kaolinite (AL.sub.2 O.sub.3 2SiO.sub.2 H.sub.2 O) or Montmorillonite (AL.sub.2 O.sub.3 4SiO.sub.2 4-7H.sub.2 O) from the cracks, crevices, and cleavage plains of used foundry sand grains and for controlling the grain distribution of the finished products from a thermal sand reclaimer.
2. Description of the Prior Art
Foundry sand is commonly used to make core molds into which ferrous and non-ferrous metals are cast. The core mold consists of sand bonded with special additives including inorganic binders such as clay, and organic resin binders such as phenol, melamine, or urea formaldehyde.
Previously, after the casting has set within the mold, the mold was broken away and discarded. Various factors, such as the depletion of natural sand deposits and the cost of disposing of used sand in accordance with recent environmental regulations, have now made it more economical and advantageous to reclaim the used sand for repeated use.
At the present state of the art, random mixtures of waste clay and resin bonded foundry sands are reclaimed for re-use in the mold and core-making processes by subjecting them to a multi-step reclamation process including: (1) Lump breaking to uniform granulation, (2) calcination in various types of furnaces, (3) post cooling in water-cooled fluid beds and (4) post scrubbing by either mechanical centrifugal or pneumatic attrition. The post scrubbing steps are necessitated due to the presence of minute traces of fused clays and metallic oxides on the sand grains which are the result of the sand grains being subjected to the high temperatures that occur at the metal mold interface during the metal pouring phase of the foundry casting process. It is these minute traces of clays and oxides that significantly reduce the strength of the acid catalyzed synthetic resin bonding systems, when thermally reclaimed sands are not post-scrubbed prior to re-use for mold and core making. Additionally, post scrubbing assures consistent control of the final screen distribution of the finished product and provides a reclaimed sand with properties and characteristics very nearly equal to the properties and characteristics of new sand, thereby assuring the rebondability of the reclaimed sand within the foundry core and molding processes.
Techniques are known in the prior art for carrying out the general steps of the multi-stage reclamation process described above. For example, U.S. Pat. No. 4,549,698--Oct. 29, 1985 to Andrews, entitled "Method of Reclaiming Foundry Sand", and assigned to the assignee of the present invention, presented a novel method for reclaiming mixtures of clay and resin bonded sands. The method utilized a triple fluid bed calcining reactor but did not specifically describe the scrubbing stage of the method. See also, U.S. Pat. No. 4,978,076, issued Dec. 18, 1990 to Andrews and Reier, entitled "Method of Separating Hazardous, Substances in Waste Foundry Sands." The method described therein utilizes a triple fluid bed thermal reactor, a second stage water-cooled sand cooling unit and a third stage pneumatic attrition scrubber. Prior art pneumatic attrition scrubbers are described in such references as U.S. Pat. Nos. 2,813,318 by Horth; 3,088,183 by McIIlivine; 3,825,190 by Kauffman; 3,907,213 by Kauffman; and 4,177,952 by Rikker.
The dry impact type scrubbers described in the above cited references are all claimed to be mechanical improvements over the prior art due to more efficient scrubbing. In all cases, only pneumatic scrubbing action of the sand grains is used for separating the fused grains, sand grain clusters, stripping off carbonaceous matter and spent binders. The sand grains are propelled out of a static bed, by a high pressure stream of air, up a vertical blast tube to impact against a layer of sand grains held against a conical target plate before falling back down into the static sand bed.
This action is repeated through as many blast tubes as are necessary to clean the sand grains. The output capacity is related to the number of cells in series installed in a unit. "Fines" are removed via the exhaust systems in an uncompensating and non-controlled manner. Although the draft fan provided on the exhaust system creates a negative pressure, this negative pressure is continually varying, being influenced by the atmospheric conditions which affect the characteristics of the air, the dust loading to the downstream fabric filter, the design of the exhaust system as well as the condition of the filtering media within the downstream fabric filter. These conditions are continually varying due to dust loading and conditions external to the filtering equipment.
The energy consumed in the prior art scrubbing systems of the type described is almost identical to the energy required for attrition reclamation of the used sand. For example, data published by National Engineering Company, specifies the following blast air requirements for scrubber units of the type under consideration:
TABLE I __________________________________________________________________________ CAPACITY 1-TPH 2-TPH 3-TPH 4-TPH 5-TPH __________________________________________________________________________ NUMBER OF CELLS 2-CELL 4-CELL 6-CELL 8-CELL 10-CELL EXHAUST CFM 1800 3000 4200 6600 9000 MOTOR HP 25 40 60 75 100 COMPRESSOR CFM (3 PSI) 1150 1800 2800 3750 4500 __________________________________________________________________________
For the total amount of work required to attrition reclaim 60% of waste clay bonded foundry sands, the cost per ton for energy consumed can perhaps be justified. However, to employ these dry scrubbing units to eliminate the minute traces of fused clay and metallic oxides from the cracks, crevices and cleavage plains of the thermally calcined sand grains introduces a significant waste of electrical energy into the overall sand reclamation process and dramatically reduces the yield from the 60% mentioned to 40%-45%.
In addition to the inefficient use of energy inherent in the scrubbing stage of existing reclamation systems, another problem exists in achieving consistent control of the final screen distribution of the finished reclaimed sand grain product. The necessary control for a consistent grain distribution (sieve analysis; screen distribution; screen analysis; all synonymous with grain distribution) is nonexistent in the presently available systems and thus produces a non-consistent finished product which is very difficult to use in a production operation on a continuous basis. This existing problem is managed, at the present time, by blending in a high percentage of new sand, a percentage in the range of 30% to 70% depending upon the severity of the problem surrounding the cleanliness of the sand and the grain distribution and most importantly, the consistency of both.
The present invention has as its object to provide a method for substantially reducing the energy required for "post" dry scrubbing calcined sand grains in a used foundry sand reclamation process to eliminate residual minute traces of fused clay and metallic oxides from the cracks, crevices and cleavage plains of the sand grains.
Another object of the present invention is to provide a continuously processing "cooler-scrubber" to closely match the output of the thermal reclamation equipment used in calcining the used foundry sand grains, thereby eliminating the cost of surge bins, conveying equipment and special batching controls.
Another object of the present invention is to eliminate the need for mechanical moving parts of the type used to control the flow of sand streams within the sand scrubbing zone of the scrubber to improve reliability of operation and down time for routine maintenance.
Another object of the invention is to automatically control the retention of silica fines in the scrubbing zone of the scrubber to ensure abrasive action in the cracks, and crevices of the sand grains to remove the minute traces of fused clays and metallic oxides.
A further object of the invention is to integrate the "post cooling unit" stage of the reclamation process with the "post dry scrubbing unit" stage to eliminate the material handling equipment between the two stages and further reduce the electrical energy required in the overall reclamation process.
Another object of the invention is to automatically control the "post dry scrubbing" stage negative atmosphere within very narrow limits to consistently remove the minute traces of fused clays and metallic oxides from the sand mass once they have been separated from the sand grains as well as remove the undesirable "fines" from the sand mass, assuring a final reclaimed product suitable for reuse in the foundry processes.
Another object of the invention is to increase the process efficiency for the recovery of a national resource and its conservation.
Another object of the invention is the reduction of waste material disposal cost and a reduction in foundry manufacturing costs.
Another object is to increase the "yield" in true silica by integrating the cooler-scrubber with the thermal process.